Redirect After Form Submit

I have a form set up and a javascript file (as shown below) that I want to validate the input and then redirect to a different website (index.html). The validation works perfectly, but I can't figure out how to get the form to redirect to the wanted page instead of just showing the post return. The form posts to a php file (which I am unable to access), and I would like the return of that file to then be displayed on index.html.